Structured Headers for HTTP

This document describes a set of data types and algorithms associated with them that are intended to make it easier and safer to define and handle HTTP header fields. It is intended for use by new specifications of HTTP header fields as well as revisions of existing header field specifications when doing so does not cause interoperability issues.¶

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Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.¶

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Specifying the syntax of new HTTP header fields is an onerous task; even with the guidance in [RFC7231], Section 8.3.1, there are many decisions – and pitfalls – for a prospective HTTP header field author.¶

Once a header field is defined, bespoke parsers and serialisers often need to be written, because each header has slightly different handling of what looks like common syntax.¶

This document introduces a set of common data structures for use in HTTP header field values to address these problems. In particular, it defines a generic, abstract model for header field values, along with a concrete serialisation for expressing that model in HTTP/1 [RFC7230] header fields.¶

HTTP headers that are defined as “Structured Headers” use the types defined in this specification to define their syntax and basic handling rules, thereby simplifying both their definition by specification writers and handling by implementations.¶

Additionally, future versions of HTTP can define alternative serialisations of the abstract model of these structures, allowing headers that use it to be transmitted more efficiently without being redefined.¶

Note that it is not a goal of this document to redefine the syntax of existing HTTP headers; the mechanisms described herein are only intended to be used with headers that explicitly opt into them.¶

To specify a header field that is a Structured Header, see Section 2.¶

Section 3 defines a number of abstract data types that can be used in Structured Headers.¶

Those abstract types can be serialised into and parsed from textual headers – such as those used in HTTP/1 – using the algorithms described in Section 4.¶

This specification intentionally defines strict parsing and serialisation behaviours using step-by-step algorithms; the only error handling defined is to fail the operation altogether.¶

This is designed to encourage faithful implementation and therefore good interoperability. Therefore, implementations that try to be “helpful” by being more tolerant of input are doing a disservice to the overall community, since it will encourage other implementations to implement similar (but likely subtly different) workarounds.¶

In other words, strict processing is an intentional feature of this specification; it allows non-conformant input to be discovered and corrected early, and avoids both interoperability and security issues that might otherwise result.¶

Note that as a result of this strictness, if a header field is appended to by multiple parties (e.g., intermediaries, or different components in the sender), it could be that an error in one party’s value causes the entire header field to fail parsing.¶

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “NOT RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in BCP 14 [RFC2119][RFC8174] when, and only when, they appear in all capitals, as shown here.¶

This document uses the Augmented Backus-Naur Form (ABNF) notation of [RFC5234], including the VCHAR, SP, DIGIT, ALPHA and DQUOTE rules from that document. It also includes the OWS rule from [RFC7230].¶

This document uses algorithms to specify parsing and serialisation behaviours, and ABNF to illustrate expected syntax in HTTP/1-style header fields.¶

For parsing from HTTP/1 header fields, implementations MUST follow the algorithms, but MAY vary in implementation so as the behaviours are indistinguishable from specified behaviour. If there is disagreement between the parsing algorithms and ABNF, the specified algorithms take precedence. In some places, the algorithms are “greedy” with whitespace, but this should not affect conformance.¶

For serialisation to HTTP/1 header fields, the ABNF illustrates the range of acceptable wire representations with as much fidelity as possible, and the algorithms define the recommended way to produce them. Implementations MAY vary from the specified behaviour so long as the output still matches the ABNF.¶

To define a HTTP header as a structured header, its specification needs to:¶

Reference this specification. Recipients and generators of the header need to know that the requirements of this document are in effect.

Specify the header field’s allowed syntax for values, in terms of the types described in Section 3, along with their associated semantics. Syntax definitions are encouraged to use the ABNF rules beginning with “sh-“ defined in this specification.

Specify any additional constraints upon the syntax of the structured used, as well as the consequences when those constraints are violated. When Structured Headers parsing fails, the header is discarded (see Section 4.2); in most situations, header-specific constraints should do likewise.

Note that a header field definition cannot relax the requirements of a structure or its processing because doing so would preclude handling by generic software; they can only add additional constraints. Likewise, header field definitions should use Structured Headers for the entire header field value, not a portion thereof.¶

# Foo-Example Header
The Foo-Example HTTP header field conveys information about how
much Foo the message has.
Foo-Example is a Structured Header [RFCxxxx]. Its value MUST be a
dictionary ([RFCxxxx], Section Y.Y). Its ABNF is:
Foo-Example = sh-dictionary
The dictionary MUST contain:
* Exactly one member whose key is "foo", and whose value is an
integer ([RFCxxxx], Section Y.Y), indicating the number of foos
in the message.
* Exactly one member whose key is "barUrls", and whose value is a
string ([RFCxxxx], Section Y.Y), conveying the Bar URLs for the
message. See below for processing requirements.
If the parsed header field does not contain both, it MUST be
ignored.
"foo" MUST be between 0 and 10, inclusive; other values MUST cause
the header to be ignored.
"barUrls" contains a space-separated list of URI-references
([RFC3986], Section 4.1):
barURLs = URI-reference *( 1*SP URI-reference )
If a member of barURLs is not a valid URI-reference, it MUST cause
that value to be ignored.
If a member of barURLs is a relative reference ([RFC3986],
Section 4.2), it MUST be resolved ([RFC3986], Section 5) before
being used.

This specification defines minimums for the length or number of various structures supported by Structured Headers implementations. It does not specify maximum sizes in most cases, but header authors should be aware that HTTP implementations do impose various limits on the size of individual header fields, the total number of fields, and/or the size of the entire header block.¶

Dictionaries are ordered maps of key-value pairs, where the keys are short, textual strings and the values are items (Section 3.5). There can be one or more members, and keys are required to be unique.¶

Implementations MUST provide access to dictionaries both by index and by key. Specifications MAY use either means of accessing the members.¶

In HTTP/1, keys and values are separated by “=” (without whitespace), and key/value pairs are separated by a comma with optional whitespace. For example:¶

Example-DictHeader: en="Applepie", da=*w4ZibGV0w6ZydGU=*

Typically, a header field specification will define the semantics of individual keys, as well as whether their presence is required or optional. Recipients MUST ignore keys that are undefined or unknown, unless the header field’s specification specifically disallows them.¶

Parsers MUST support dictionaries containing at least 1024 key/value pairs, and dictionary keys with at least 64 characters.¶

In HTTP/1, each inner-list is separated by a comma and optional whitespace, and members of the inner-list are separated by semicolons and optional whitespace. For example, a header field whose value is defined as a list of lists of strings could look like:¶

Example-StrListListHeader: "foo";"bar", "baz", "bat"; "one"

Header specifications can constrain the types of individual inner-list values if necessary.¶

Parsers MUST support lists of lists containing at least 1024 members, and inner-lists containing at least 256 members.¶

Parameterised Lists are arrays of parameterised identifier with one or more members.¶

A parameterised identifier is a token (Section 3.9}) with an optional set of parameters, each parameter having a textual name and an optional value that is an item (Section 3.5). Ordering between parameters is not significant, and duplicate parameters MUST cause parsing to fail.¶

When a receiving implementation parses textual HTTP header fields (e.g., in HTTP/1 or HTTP/2) that are known to be Structured Headers, it is important that care be taken, as there are a number of edge cases that can cause interoperability or even security problems. This section specifies the algorithm for doing so.¶

Given an ASCII string input_string that represents the chosen header’s field-value, and header_type, one of “dictionary”, “list”, “list-list”, “param-list”, or “item”, return the parsed header value.¶

Discard any leading OWS from input_string.

If header_type is “dictionary”, let output be the result of Parsing a Dictionary from Text (Section 4.2.1).

If header_type is “list”, let output be the result of Parsing a List from Text (Section 4.2.3).

If header_type is “list-list”, let output be the result of Parsing a List of Lists from Text (Section 4.2.4).

If header_type is “param-list”, let output be the result of Parsing a Parameterised List from Text (Section 4.2.5).

If header_type is “item”, let output be the result of Parsing an Item from Text (Section 4.2.7).

Discard any leading OWS from input_string.

If input_string is not empty, fail parsing.

Otherwise, return output.

When generating input_string, parsers MUST combine all instances of the target header field into one comma-separated field-value, as per [RFC7230], Section 3.2.2; this assures that the header is processed correctly.¶

For Lists, Lists of Lists, Parameterised Lists and Dictionaries, this has the effect of correctly concatenating all instances of the header field, as long as individual individual members of the top-level data structure are not split across multiple header instances.¶

Strings split across multiple header instances will have unpredictable results, because comma(s) and whitespace inserted upon combination will become part of the string output by the parser. Since concatenation might be done by an upstream intermediary, the results are not under the control of the serialiser or the parser.¶

Integers, Floats and Byte Sequences cannot be split across multiple headers because the inserted commas will cause parsing to fail.¶

If parsing fails – including when calling another algorithm – the entire header field’s value MUST be discarded. This is intentionally strict, to improve interoperability and safety, and specifications referencing this document cannot loosen this requirement.¶

Note that this has the effect of discarding any header field with non-ASCII characters in input_string.¶

Given an ASCII string input_string, return a byte sequence. input_string is modified to remove the parsed value.¶

If the first character of input_string is not “*”, fail parsing.

Discard the first character of input_string.

If there is not a “*” character before the end of input_string, fail parsing.

Let b64_content be the result of removing content of input_string up to but not including the first instance of the character “*”.

Consume the “*” character at the beginning of input_string.

If b64_content contains a character not included in ALPHA, DIGIT, “+”, “/” and “=”, fail parsing.

Let binary_content be the result of Base 64 Decoding [RFC4648] b64_content, synthesising padding if necessary (note the requirements about recipient behaviour below).

Return binary_content.

Because some implementations of base64 do not allow reject of encoded data that is not properly “=” padded (see [RFC4648], Section 3.2), parsers SHOULD NOT fail when it is not present, unless they cannot be configured to do so.¶

Because some implementations of base64 do not allow rejection of encoded data that has non-zero pad bits (see [RFC4648], Section 3.5), parsers SHOULD NOT fail when it is present, unless they cannot be configured to do so.¶

This specification does not relax the requirements in [RFC4648], Section 3.1 and 3.3; therefore, parsers MUST fail on characters outside the base64 alphabet, and on line feeds in encoded data.¶

The size of most types defined by Structured Headers is not limited; as a result, extremely large header fields could be an attack vector (e.g., for resource consumption). Most HTTP implementations limit the sizes of size of individual header fields as well as the overall header block size to mitigate such attacks.¶

It is possible for parties with the ability to inject new HTTP header fields to change the meaning of a Structured Header. In some circumstances, this will cause parsing to fail, but it is not possible to reliably fail in all such circumstances.¶

Earlier proposals for structured headers were based upon JSON [RFC8259]. However, constraining its use to make it suitable for HTTP header fields required senders and recipients to implement specific additional handling.¶

For example, JSON has specification issues around large numbers and objects with duplicate members. Although advice for avoiding these issues is available (e.g., [RFC7493]), it cannot be relied upon.¶

Likewise, JSON strings are by default Unicode strings, which have a number of potential interoperability issues (e.g., in comparison). Although implementers can be advised to avoid non-ASCII content where unnecessary, this is difficult to enforce.¶

Another example is JSON’s ability to nest content to arbitrary depths. Since the resulting memory commitment might be unsuitable (e.g., in embedded and other limited server deployments), it’s necessary to limit it in some fashion; however, existing JSON implementations have no such limits, and even if a limit is specified, it’s likely that some header field definition will find a need to violate it.¶

Because of JSON’s broad adoption and implementation, it is difficult to impose such additional constraints across all implementations; some deployments would fail to enforce them, thereby harming interoperability.¶

Since a major goal for Structured Headers is to improve interoperability and simplify implementation, these concerns led to a format that requires a dedicated parser and serialiser.¶

Structured headers intentionally limits the complexity of data structures, to assure that it can be processed in a performant manner with little overhead. This means that work is necessary to fit some data types into them.¶

Sometimes, this can be achieved by creating limited substructures in values, and/or using more than one header. For example, consider:¶

Since the description contains a list of key/value pairs, we use a Parameterised List to represent them, with the token for each item in the list used to identify it in the “descriptions” member of the Example-Thing header.¶

When specifying more than one header, it’s important to remember to describe what a processor’s behaviour should be when one of the headers is missing.¶

If you need to fit arbitrarily complex data into a header, Structured Headers is probably a poor fit for your use case.¶

A generic implementation should expose the top-level parse (Section 4.2) and serialise (Section 4.1) functions. They need not be functions; for example, it could be implemented as an object, with methods for each of the different top-level types.¶